Process for the recovery of nitric acid



Patented May 10, 1932 UNITED- STATES GHRISTOPH BECK, OF LUDWIGSHAIEN-ON-THE-BHINE, "AND HELMUT WEISSBAGH, OI!" MANNHEIM, GERMANY, ASSIGNORS TO' I. G. FARBENINDUSTRIE AKTIENGESELL-fi PATENT-(OFFICE!7 SCHAFT, OF FRANKFORT ON THE MAIN, GERMANY, A CORPORATION OF'GERM' ANY PROCESS FOR THE RECOVER? or mm cm The present invention relates to the process for the recovery of nitric acid.

Nitric acid cannot directly be recovered in a pure state, by distillation or. other physical methods of separation, from solutions still .containing other volatile acids, especially halogen hydrides. This is not only due to V i the volatility of the difi'erent acids, but above all also due to the circumstances that, for ex ample when ,haloge'n hydrides are present,

volatile nitrosyl compounds of'thehalogens, are formed, partly already in the solutions containing nitricacid, partly duringthe distillation or'condensation, which nitrosyl com- I pounds may, under certain circumstances, be recondensed only with difliculty and give rise to considerable losses of nitrogen.

We have now found that a complete separation of thecompone-nts is effected in an advantageous manner, with the recovery of pure nitric acid or pure salts thereof from solutions containing other'volatile acids or their salts in addition to nitric acid, by converting the nitric acid present in the solution by treating the solution with a gaseous re ducing agent into'nitrogen oxides, and oxidizing these to nitric acid, it deslred, after washing with water or alkaline substances.

The process can be carried out, for example, I by first treating the solution, which may con-J tain still other substances, such as salts or non-volatile acids, 'in addition to nitric acid and one or more other volatile acids, with reducing'agents, such as sulphur dioxide, hy-

drogen sulphide and the like or gaseous mixtures containing the same, thereby reducing the nitriciacid'to lower nitrogen oxides. In

this case, it is advisable toselect' such condi: tions of reaction which efi'ect, preferably,the

formation of nitric oxide, because this, on

*acco unt of its low solubility, can be very easily expelled fromthe solution. In some l' cases,fhowever, it is also advantageous toconduct the reductionin such a waythatnitroi gen trioxide or tetroxide is mainly produced, whereby, when oxidizing same tonitric acid,

reducing gases which may alsocontain oxygen, for example air or steam, through the No Drawing. Application filedlDecember 19,1928, Serial No, 327,177, and in Germanype cember 3 1, 1927.

it is advisable to'free the resulting nitrogen oxides, especially in thereduction to nitric .0Xide,'from extraneous Volatile acids carried along, prior to the oxidation. For this purpose washing with'water will often be sufiiclent; but, if the nitric acid is'to'be obtained entirelyfree'from extraneous acids, the nitric oxide may be treated with alkaline agents instead of or in addition to the washing with water. In order to oxidize the nitrogen 0xides, these are mixed with the required quanof oxygen or air, and are worked up to nltric acid, or nltrites, or nitrates, in the known manner, for example by absorption with acid or alkaline agents.

The aforedescri'bed process can be advantageouslyfcombined with the production of alkalimetal nitrate.

sium' chloride, for example, with nitric acid, the resulting hydrochloric acid being removed by distillation, as for example'in 'vacuo or bya current of inert gas. According to this method of working, however, losses of nltrogen are unavoidable, sincej nitrogen compounds are carried away from the solu tion at thesame time as the hydrochloric acid.

' Moreover, -the said "process also has the disaforedescribed process "for the recovery of.

nitric acid or nitrates or nitrites withthe production of alkali metal nitrates by converting an alkali metal chloride or sulphate ora mixture of both with nitric acid and, after the conversion and separation of the nitrate, deposited, if desired, by cooling,treat ing the residual mother liquor with a gaseous reducing agent, the nitrogen oxides expelled from the solution being then converted into nitric acid which is again converted with alkali metal chloride or sulphate to form It is already known that alkali metal nitrate can be produced by converting potas-' alkali metal nitrate. be carried out, for example, in the following manner:

The conversion of the alkali metal salt with nitric acid is effected at the ordinary or at elevated .temperature. resulting-deposit of nitrate obtained, if desired, b cooling the reaction solution, is separate :By treating the filtrate; withxa. gaseous rey .ducing.agent,the nitric acid; remainingin the. mother liquor is converted into nitrogen-oxygen compounds in a lower-stage of oxidation,

v chiefly nitric oxide, which, after admixture with oxygen or air, are reconverted'intonitric acid by any known and. suitable. mann er, .as,,

fonexampleby absorption inacid solution Qr, by ,liquefaetion., I The. recovered nitric acid gisaused again, and employed for. converting furth'er. quantities of]. alkali .metal chloride q .or. sulphateI .It is'advantageous to treat the,

' ,motherliquonfwhich has been freed from the iiiiitrogenioxides, for the recoveryQof the al-i ,.kali contained therein, byprecipitatingit in.

' fu -formed by. the reduct on treatment, such as .the form ofthealkali metal salt, of theacid.

,. sulphate or bisulphate. This.can be 'efiect ed by .simply cooling the mother liquor, and

inlsome cases the procedure may consist, for example, in'distilling'ofi. the whole or a por-L .Ition of..the.hydrochloric acid resulting from 1 theconversion. of the alkali metal chloride,

i i and then precipitating thealkali from the residual solution, ifdesired, by cooling, in the a5, g i formed by the reduction treatment.'- Losses. ofnitrogen areprevented by this mode" of Qw'orkifigQfiThe recovered. alkali metal com-1',

form... of .fchlorides or as the salt ofthe acid pound may be again subjected, together with ,conyerted; into neutral sulphate. By work- T j the 1alkali .andinitric acid employed is con- .verted. into alkali metal nitrate.

. T". In the conversion of alkali 'nieta'l sulphate ,tion,.' of .alk'a-li metal nitrate, and sulphuric with nitric acid, a'colnparajtively' weak soluac'i'd is obtained audits-separation by distilfilation would be attended with considerableexpense owing to the, large quantityof water to -be evaporated. Consequently, removing the' nitric acid, by reduction with a suitable, i reducing agent, such asfsulphur dioXide,-ac-- cording 'to the process, herein described, re- :su1ts, in addition to the separation of the acids without evaporating,-in extensivelyconcentra'ting. the sulphuric acid, so. that the whole,

or the great-(apportion of the alkalipresentj'ot air for the formationlof nitrieacid, are

in the solution separates out in the 'form of Moreover, the process lierein 'described The said process may from the liquid: byyfiltration', :or, other;like n1eans. -g

possesses over the usual. separation of the nitric acid by-distillation the advantage of preventing the nitric acid from becoming enriched with hydrochloric acid which is usuallypresent in the form of its saltsin commercial alkali metal sulphate. In theseparation of the mixture of nitrate and sulphuric acid according to the present process, the hydro- ;chlori c acid remains in-uthei sulphuric acid solution, which. isnnotjreturned tothe process, and, moreoverpan, be easily freed from the hydrochloric acid by distillation.

- The following-examples will further illustrate the nature of the saidginvention -which, however, is not limited thereto;

Ewample 1 l0OQ,kilograms of a solution. containing 1 .k1lograms of hydrochloric acidand 100 kilograms of nitric acid areheated tdfiOfi and treated, in astifrring. vessel,'.,with,.55 cubic meters in all of 98 percentofisulphurdioxide. The escaping nitrogen oxide gasesare'passed through a reflux condenser mounted on the stirringfvessel, then through a; tower throughr qwhich water trickles and-which, is charg ed Qwith. fillers,.. and finally: through a washing apparatus charged with milk of, lime. The nitrogen oxide gases ,thus purified Q are i then {mined with. about 150 cubic meters, iii all-,

of air and conducted tofan; apparatusiforjan ac d absorption. About .95fkilograms of nitr1c, acid are obtained asan acid completely [free from chlorinel} In the initial solution remains nearly the whole.ofthe hydrochloric acid as wellas thesulPhuric -acid, forn1ed from the added sulphur ,dioxide,1whi ch can be concentrated by evaporation and at thesame .time freed from the hydrochloric acid.

" Example 2 i125 kilograms of potassiur'nf bisulphate and '60() k lograms of ground potassiumchloride separated from the residual. motlier liquor About 750 ki-logramslof nitrate, containing nitrate; The mother liq-nor, is-treated at, about 50 centigrade with the gases obtainedby roasting ,pyrites, the treatment being ca rried onin astirring vessel or-scrubbingtoweruntil 7 denitration is complete, The efliuent-nis troge'n oxides are washed ,with% water,..and,-- after being mixed with the requisite amount fp Q g' nab rp ion. apparatus. -bl l lp a e, w t outiul'th r ,0ub1 n c001 This treatment results, .in' the recovery. of

f about25 kilograms oi nitrogen in the form filtration by suction orI centrifuging:

95 per centof potassium nitrate areobtaiiied which canbe,still further purified by purgmg. with a'saturated solution of potassium of nitric acid, which can be converted into potassium nitrate by renewed treatment with potassium chloride. The whole ofthe hydrochloric acid, about 800 kilograms, is recovered, in the form of 20 per cent acid, from the denitrated mother liquor by evaporating in a column apparatus. The evaporation residue, on being cooled to ordinary temperature, deposits about kilograms of potassium bisulphate, which is subjected to renewed conversion with nitric acid.

What we claim is 1. The process for the recovery of nitric acid which comprises treating a solution containing nitric acid and another volatile acid with a gas comprising sulphur dioxide and oxidizing the nitric oxides liberated.

2. The process for the recovery of nitric acid which comprises treating a solution containing nitric acid, another acid and salts of these acids with a gas comprising sulphur dioxide and oxidizing the nitric oxides liberated.

3. The process for the recovery of nitric acid which comprises treating a solution containing nitric acid, an alkali metal salt and free acid derived from this salt with a gascomprising sulphur dioxide and oxidizing the nitric oxides liberated.

4. The process for the recovery ofnitric acid which comprises treating a solution containing nitric acid, an alkali metal halide, an alkali metal sulphate, free halogen hydride and sulphuric acid with a gas comprising sulphur dioxide and oxidizing the nitric oxides liberated.

5. The process for the recovery of nitric acid which comprises treating a solutioncontaining nitric acid and potassiumchloride with a gas comprising sulphur dioxide and oxidizing the nitric oxides liberated.

In testimony whereof we have hereunto set our hands.

CHRISTOPH BECK. HELMUT WEISSBACH. 

